Method for producing n-phosphonomethylglycine

ABSTRACT

The present invention relates to a process for the recovery of N-phosphonomethylglycine from an aqueous mixture which contains N-phosphonomethylglycine, ammonium halides, and alkali metal or alkaline earth metal halides and optionally organic impurities in dissolved form, where  
     (a) the pH of the mixture is adjusted to a value in the range from 2 to 8,  
     (b) a separation of the mixture on a selective nanofiltration membrane is performed, a retentate which is richer in N-phosphonomethylglycine and poorer in halides and a permeate which is richer in halides and poorer in N-phosphonomethylglycine being obtained, and  
     (c) the N-phosphonomethylglycine is recovered from the retentate.  
     The process according to the invention makes possible the recovery of the N-phosphonomethylglycine with simultaneous separation of the halide salts.

[0001] The present invention relates to a process for the recovery ofN-phosphonomethylglycine from a mixture which containsN-phosphonomethylglycine, chloride salts and optionally organicimpurities.

[0002] N-Phosphonomethylglycine (glyphosate) is a total herbicide whichis employed to a great extent. Numerous processes for the preparation ofphosphonomethylglycine are known. In one of the most customaryprocesses, the last stage consists in oxidizingphosphonomethyliminodiacetic acid or a salt thereof catalytically usingair, oxygen or hydrogen peroxide. The oxidation is generally carried outin aqueous medium using carbon, molybdenum or vanadium salts, platinum,rhodium or palladium etc. as catalysts. In addition toN-phosphonomethylglycine, carbon dioxide and formaldehyde are formedhere. A process of this type is described in U.S. Pat. No. 3,950,402 andU.S. Pat. No. 3,969,398.

[0003] Both the reaction mixture and the mother liquor which areobtained after recovery of the N-phosphonomethylglycine from thereaction mixture by crystallization thus contain formaldehyde, which isknown to be carcinogenic. In IT 1281094 it is therefore proposed toremove the formaldehyde with the aid of a separation process using aselective membrane having a pore size in the range from 10 to 1000 nm.The formaldehyde is removed here via the permeate.

[0004] PCT/EP00/13162 describes a process for the preparation ofN-phosphonomethylglycine, where a hexahydrotriazine derivative of theformula IIa

[0005] in which X, in particular, is CN, is reacted with a triacylphosphite of the formula P(OCOR³)₃, in which R³, in particular, isphenyl, and the product obtained is hydrolyzed. TheN-phosphonomethylglycine is recovered from the reaction mixture bycrystallization at pH 0.5 to 2. The mother liquor has a completelydifferent composition than the mother liquor obtained in the oxidationof phosphonomethyliminodiacetic acid. It additionally containsconsiderable amounts of N-phosphonomethylglycine, small amounts ofaminomethylphosphonic acid, glycine and bis(phosphonomethyl)glycine, andlarge amounts of chloride salts. It would be desirable to minimize thelosses of valuable substance and the pollution of the waste water.

[0006] The present invention is therefore based on the object of makingavailable an economical process for the recovery ofN-phosphonomethylglycine from an aqueous mixture which containsN-phosphonomethylglycine, ammonium salts, alkali metal or alkaline earthmetal salts and optionally organic impurities in dissolved form. At thesame time, the process should make possible a recovery of theN-phosphonomethylglycine which is as complete as possible and a recoveryof the ammonia contained in the ammonium salts which is as complete aspossible.

[0007] Surprisingly, it has now been found that this object is achievedif the aqueous mixture mentioned is subjected to a separation in apressure-driven separation process using a selective nanofiltrationmembrane.

[0008] The present invention therefore relates to a process for therecovery of N-phosphonomethylglycine from an aqueous mixture whichcontains N-phosphonomethylglycine, ammonium halides, alkali metal oralkaline earth metal halides and optionally organic impurities indissolved form, where

[0009] a) the pH of the mixture is adjusted to a value in the range from2 to 8,

[0010] b) a separation of the mixture on a selective nanofiltrationmembrane is performed, a retentate which is enriched withN-phosphonomethylglycine and depleted in halides and a permeate which isenriched in the halides and depleted in N-phosphonomethylglycine beingobtained, and

[0011] c) N-phosphonomethylglycine is recovered, if desired, from theretentate.

[0012] FIG. 1 shows a schematic representation of step b of the processaccording to the invention in a continuous procedure

[0013] FIG. 2 shows a schematic representation of step b of the processaccording to the invention in a batchwise procedure

[0014] FIG. 3 shows a schematic representation of step b of the processaccording to the invention as described in FIG. 1 with subsequentdiafiltration

[0015] FIG. 4 shows a schematic representation of step b of the processaccording to the invention in the implementation as a diafiltration

[0016] FIG. 5 shows a schematic representation of step b of the processaccording to the invention in a three-stage embodiment.

[0017] N-Phosphonomethylglycine can be present in different ionic formsas a function of the pH. All these forms are included according to theinvention.

[0018] The mixture used as a starting material is an aqueous mixture. Itcan optionally contain a small proportion, in particular up to 10% byweight, of a water-miscible solvent, for example a mono- or polyalcohol,such as methanol, ethanol, isopropanol, glycol, 1,2- or 1,3-propanedioletc, acetone, dioxane or tetrahydrofuran.

[0019] The mixture contains, based on the weight ofN-phosphonomethylglycine, an excess of ammonium halides and alkali metalor alkaline earth metal halides. The ammonium halides, in addition toNH₄Hal, are also to be understood as meaning ammonium halides which aresubstituted by 1 to 3 C₁-C₄-alkyl or benzyl groups, e.g. methyl-,dimethyl-, trimethyl-, tributyl- and benzylammonium halide. Preferredalkali metal or alkaline earth metal halides are sodium, potassium andcalcium halides. Chlorides are preferred as halides. The amount ofhalides is, in general, at least 8% by weight, in particular at least10% by weight and particularly preferably at least 12% by weight, basedon the total weight of the mixture. The amount ofN-phosphonomethylglycine is in general less than 4% by weight and inparticular less than 3% by weight. It is preferably in the range from0.5 to 3.0% by weight.

[0020] According to a preferred embodiment, the mixture originates fromthe preparation of N-phosphonomethylglycine, in which ahexahydrotriazine derivative of the formula II

[0021] in which X is CN or CONR¹R², where R¹ and R² can be identicaldifferent and are H or C₁-C₄-alkyl, is reacted with a triacyl phosphiteof the formula III

P(OCOR³)₃

[0022] in which the radicals R³, which can be identical or different,are C₁-C₁₈-alkyl or aryl which is optionally substituted by C₁-C₄-alkyl,NO₂ or OC₁-C₄-alkyl, and the product obtained is hydrolyzed. Thereaction can be carried out with or without solvent. An inert organicsolvent is preferably used, in particular a halogenated hydrocarbon,such as 1,2-dichloroethane. The reaction components are expedientlyemployed in essentially stoichiometric amounts. The reaction temperatureis in general in the range from −10° C. to 140° C.

[0023] An acidic hydrolysis is then carried out, a hydrohalic acid, suchas hydrochloric acid, in particular being used. When using hydrochloricacid, an ammonium chloride is then contained in the aqueous mixture usedas a starting material for the process according to the invention.

[0024] The acid is in general employed in an excess, in particular in anamount of at least 2 equivalents. The temperature at which thehydrolysis is carried out is in general in the range from approximately10 to 180° C.

[0025] The phosphonomethylglycine obtained in the hydrolysis using anexcess of acid is dissolved in the aqueous phase. The carboxylic acidR³COOH formed in the hydrolysis in general precipitates and is separatedin a customary manner.

[0026] The phosphonomethylglycine can be precipitated by adjusting theaqueous phase to a pH in the range from 0.5 to 2.0 and recovered in acustomary manner. The adjustment of the pH is carried out by addition ofan alkali metal or alkaline earth metal hydroxide, in particular byaddition of NaOH or KOH.

[0027] The mixture remaining after the recovery of thephosphonomethylglycine is employed as a starting material for theprocess according to the invention. If it contains an organic solvent,this is essentially removed from the mixture by distillation. Themixture is an aqueous solution essentially having the followingcomposition (in each case based on the total weight of the mixture):

[0028] N-phosphonomethylglycine 0.5 to 3% by weight in particular 0.5 to2.5% by weight

[0029] aminomethylphosphonic acid 0.01 to 0.5% by weight glycine 0.1 to0.4% by weight

[0030] bis(phosphonomethyl)glycine 0.2 to 0.8% by weight

[0031] ammonium halides/alkali metal or alkaline earth metal halides 10to 25% by weight, in particular 12 to 20% by weight.

[0032] The process for the preparation of N-phosphonomethylglycine viathe hexahydrotriazine of the formula II is detailed in PCT/EP00/13162.The contents of this application are also included in the presentapplication by way of reference.

[0033] In step (a) of the process according to the invention, theaqueous mixture is adjusted to a pH in the range from 2 to 8, inparticular 2.5 to 6.5, and particularly preferably to approximately 4 to5. This adjustment is carried out using a suitable acid or base, such ashydrochloric acid, sulfuric acid, phosphoric acid, sodium hydroxide orpotassium hydroxide.

[0034] In step (b) of the process according to the invention, themixture is then subjected to a separation on a selective nanofiltrationmembrane. Nanofiltration is understood as meaning, like ultrafiltrationand reverse osmosis, a pressure-driven membrane separation process usinga positive osmotic pressure on one side of the membrane, which is abovethe osmotic pressure of the solution to be treated (where the osmoticpressure is in particular determined by the retention behavior of themembrane). Nanofiltration membranes are in general applied formechanical reasons to a mono- or multilayer substructure as support madeof the same material as or a different material than the membrane. Theseparating layers can consist of organic polymers, ceramic, metal orcarbon. Frequently, membranes made of crosslinked aromatic polyamidesare used. Preferred membranes according to the invention have anionicgroups, for example sulfonic acid groups, and therefore exhibit anegative charge. Suitable nanofiltration membranes are known to theperson skilled in the art and commercially obtainable. Examples are theDesal-5 membranes of Desalination Systems, Escondido, USA, the NFmembranes of the Filmtec Corp., Minneapolis, USA (e.g. NF 70, NF 50, NF40 and NF 40HF), SU 600 membrane of Toray, Japan, and the NTR membranesof Nitto Electric, Japan (e.g. NTR 7450 and NTR 7250 membranes), cf. WO96/33005.

[0035] Preferred membranes according to the invention have separationlimits of from 50 to 1000 D, preferably 100 to 500 D.

[0036] In practice, the nanofiltration membranes are incorporated into‘membrane modules’ (membrane units). All module geometries which aremechanically stable under the temperature and pressure conditions of theprocess according to the invention are suitable here. Flat, tubular,multichannel element, capillary or wound geometry, for example, aresuitable. Appropriate embodiments are supplied commercially.

[0037] The temperature at which step (b) is carried out is not critical.In general, the temperature is chosen such that damage to the membraneis avoided. In the case of polymer membranes, the process is thereforeexpediently carried out at a temperature of less than 50° C. Ceramic ormetal membranes, however, can also be employed at higher temperature.

[0038] Step (b) is a pressure-driven separation process. The drivingforce for the separation is thus a pressure difference over themembrane, which must be greater than the osmotic pressure of the mixtureemployed. Expediently, the process is carried out at a transmembranepressure between retentate side and permeate side of 30 to 100 bar.

[0039] To avoid concentration polarization on the feed side of themembrane and deposition of crystallized components, the feed solution isgenerally passed over the membrane in crossflow. The retentate is atleast partially recycled here. Preferably, the process is carried out ata flowing-over rate of the feed solution in the range from 0.1 to 5 m/s.The flowing-over rate depends on the module geometry and can bedetermined in a simple manner by the person skilled in the art.

[0040] Step (b) leads to a separation of the mixture employed into aretentate, which is enriched with N-phosphonomethylglycine in comparisonto the feed solution, and a permeate, which is enriched with theammonium halide, alkali metal or alkaline earth metal halide.Surprisingly, it is thus possible to separate N-phosphonomethylglycineand the halides mentioned, although the molecular weight difference ofthe components to be separated is not very high and theN-phosphonomethylglycine is not quantitatively present in salt form.

[0041] Step (b) of the process according to the invention can berealized in different embodiments, depending on the desired degree ofrecovery of N-phosphonomethylglycine and depending on the desired degreeof depletion of the halides. Thus, step (b) can be carried out as a pureconcentration stage, in which the retentate enriched inN-phosphonomethylglycine and the permeate enriched in halides areobtained. Here, the depletion of the halides on the retentate side ofthe membrane is limited by the osmotic pressure.

[0042] If a continuing depletion of the halides in the retentate isdesired, at least one diafiltration stage follows the concentrationstage. For this purpose, just as much water is fed to the retentate ofthe concentration stage as permeate has been separated. The diluteretentate is then fed to the diafiltration stage, which is carried outunder the same conditions and using the same membrane as describedabove. In this way, a retentate which is depleted in halides isobtained, which, if desired, can be concentrated in a further step, forexample by distilling off the water.

[0043] In order to obtain a high depletion of halides, step (b) can alsobe carried out as a pure diafiltration stage. Here, the solutionobtained from stage (a) is separated as described above, where, however,just as much water is fed to the retentate as is separated as permeate.A retentate is obtained which is concentrated for the recovery ofN-phosphonomethylglycine, for example by removing the water by means ofdistillation or reverse osmosis.

[0044] The pure concentration, the concentration with subsequentdiafiltration and the pure diafiltration can all be carried out in onestage or a number of stages. In the multistage procedure, the permeateof the preceding stage is fed to a subsequent stage and the retentate ofthe subsequent stage is fed into the feed solution of the precedingstage. Here, the individual stages can be equipped with the samemembrane or with different membranes. With the multistage procedure, abetter retention of N-phosphonomethylglycine or the salts thereof isachieved.

[0045] The retentate obtained after step (b) can be used further assuch. Depending on concentration and purity, it can be subjected to aconcentration or purification or disposed of. In general, however, theretentate is treated further as in step (c) for the recovery of theN-phosphonomethylglycine contained therein.

[0046] Step (b) of the process according to the invention is illustratedbelow with the aid of the figures:

[0047] For the continuous separation of the mixture, the feed solution Fis fed in a crossflow procedure through a line 1 to a membrane unit Mhaving a housing 2 in which a membrane 3 is situated. The feed solutionF flows over the membrane 3, the retentate R being removed via line 4.If desired, some of the retentate R is fed back via line 5. The permeateP is removed via line 6.

[0048] FIG. 2 shows the batchwise separation of a feed solution F, whichis fed to a circulation container 7 via line 1. The original liquidlevel is indicated by 8. The feed solution F is fed in a crossflowprocedure to the membrane unit M described in FIG. 1 via line 9. On themembrane 3, a separation into a retentate, which is fed back into thecirculation container 7 via line 4, and a permeate P, which is removedvia line 6, takes place. In this way, concentration of the feed solutionF, which is finally removed as retentate R via line 10, occurs. Theliquid level after concentration is indicated by 11.

[0049] FIG. 3 shows the separation of a mixture in a concentration stageof a membrane unit M1 and a subsequent diafiltration stage of a membraneunit M2. The concentration stage is operated as described above inconnection with FIG. 1, a permeate P1 being removed via line 6. Theretentate is removed via line 4, diluted with water and fed to thediafiltration stage M2 of the membrane module 12 with membrane 13. Thisis likewise operated as described above with regard to FIG. 1. Aseparation into a retentate R, which is led off via line 14, a morehighly concentrated permeate P1 and a less concentrated permeate P2,which is led off via line 15, is finally obtained.

[0050] The carrying-out of step (b) as a pure diafiltration isillustrated in FIG. 4. The feed solution F is fed to the membrane unit Mvia line 1 after dilution with water and separated as described above inconnection with FIG. 1.

[0051] FIG. 5 shows schematically the process course of a three-stageseparation process. Three membrane units M1, M2 and M3 connected inseries are provided, which are equipped with a membrane 3. The number ofmembrane units can be increased as desired.

[0052] The feed solution F is fed via line 1 to the first membrane unitM1 in a crossflow procedure. A separation into a retentate R1 and apermeate P1 takes place. The retentate R1 is partially expelled via line4 and partially fed back into the feed solution via line 5. The permeateP1 is fed back in a crossflow procedure via line 6 to the secondmembrane unit M2. A separation into a retentate R2 and a permeate P2takes place. The retentate R2 is fed to the feed solution F, which isfed into the membrane unit M1 via line 1. If desired, some of theretentate R2 is fed back into the feed solution of the membrane unit M2.The permeate P2 is fed into the third membrane unit M3 in a crossflowprocedure. The retentate R3 obtained in the separation is fed to thefeed of the membrane unit M2. If desired, some of the retentate R3 isfed back into the feed solution of the membrane unit M3. The permeate P3is expelled.

[0053] In step (c) of the process according to the invention therecovery of the N-phosphonomethylglycine from the retentate obtained instage (b) takes place. For this purpose, the pH of the retentate isadjusted to 0.5 to 2.0, in particular 0.8 to 1.5, by addition of anacid, for example hydrochloric acid or sulfuric acid. If desired, theretentate is concentrated, for example by distillation or reverseosmosis. It is also possible to add precipitation aids in order toprecipitate the phosphonomethylglycine as completely as possible. Theprecipitation aid used is preferably a water-miscible solvent, such asmethanol, ethanol, isopropanol, acetone etc. The solvents can berecovered from the mother liquor by distillation and reused.

[0054] The phosphonomethylglycine is obtained in crystalline form. Itcan be recovered in a customary manner, for example by filtration.

[0055] The permeate obtained in step (b) can be disposed of or fed tofurther processing. Preferably, it is rendered alkaline, e.g. to pH13-14, using a strong base, in order to recover the ammonia or thecorresponding amine from the ammonium halides contained in the permeate.Suitable bases are alkali metal or alkaline earth metal hydroxides, suchas sodium hydroxide, potassium hydroxide or calcium hydroxide. Theammonia released in this way can be recovered, for example bydistillation or stripping with an inert gas, such as air or nitrogen.

[0056] The process according to the invention or each stage taken per secan be carried out continuously, batchwise or as a semi-batch process.

[0057] The advantages of the process according to the invention lie inthe concentration of the N-phosphonomethylglycine and thus in anincrease in yield in the preparation. Moreover, a separation of the cropprotection components from the wastewater is achieved. Finally, aseparation of the chlorides contained in the starting mixture takesplace, which makes possible a simpler recovery of ammonia from theammonium halides.

[0058] The following examples illustrate the invention withoutrestricting it.

EXAMPLE 1

[0059] In a 2 l stirring flask having a Teflon blade stirrer and refluxcondenser, 284 g of ammonium benzoate are introduced into 1000 ml of1,2-dichloroethane and 91.5 g of phosphorus trichloride are addeddropwise under a nitrogen atmosphere in the course of 30 min. Thetemperature rises during the course of this to a maximum of 36° C. Themixture is then stirred at 25 to 36° C. for a further 30 min. The batchis filtered through a pressure suction filter and the filter cake iswashed a further two times with 500 g of dichloroethane each time undernitrogen (2054 g of filtrate).

[0060] In a 2 l stirring flask having a Teflon blade stirrer and refluxcondenser, the filtrate is introduced at room temperature and thehexahydrotriazine IIa (X═CN) (45.54 g) is added. The mixture is heatedto 80° C. with stirring in the course of 30 min and stirred at 80° C.for a further 30 min. The solution is allowed to cool and hydrolyzeddirectly following this.

[0061] To this end, the substances employed are metered at 130° C. and 8bar into a tubular reactor (volume about 600 ml) having a preconnectedstatic mixer (1265 g/h of the dichloroethane solution from the precedingstage, 207 g/h of 20% strength HCl). The residence time is 30 min. Aforerun is discarded. For further processing, the two-phase mixtureobtained is collected for 60 min. The phases are separated at 60° C. andthe water phase is extracted twice with 100 g of dichloroethane eachtime.

[0062] In a round-bottomed flask having a Teflon blade stirrer, thedichloroethane still contained in the water phase is firstly stripped at60° C. by passing in nitrogen for one hour. The pH is then adjusted topH=1.0 at 40 to 60° C. in the course of 15 min using 50% strength sodiumhydroxide solution. The resulting suspension is stirred at 40° C. for afurther 3 h, allowed to cool to room temperature and the precipitatedproduct is filtered off with suction and subsequently washed with 150 gof ice water. The solid obtained is dried at 70° C. and 50 mbar for 16h.

[0063] Yield: 54.6 g of phosphonomethylglycine (purity 96.2% accordingto HPLC), corresponding to 80% yield, based on PCl₃.

[0064] The mother liquor from the crystallization has the followingcomposition: 2.10% by weight of N-phosphonomethylglycine 0.10% by weightof aminomethylphosphonic acid 0.20% by weight of glycine 0.45% by weightof bis(phosphonomethyl)glycine 16.70% by weight of  NaCl/NH₄Cl

[0065] The pH of the mother liquor is subsequently adjusted to 4 using50% strength NaOH. The mother liquor is then concentrated batchwise in astirred pressure cell. The stirred pressure cell is equipped with ananofiltration membrane of the type Desal 5 DK from Desal-Osmotics. Thenanofiltration is carried out at 40° C. and 80 bar. The mean specificpermeate flow is 5.29 kg/m²h. A retention of N-phosphonomethylglycine of99.22% and a depletion of chloride salts of 86.26% results. The resultsare compiled in table 1 below. TABLE 1 Mother liquor Retentate PermeateAmount (g) 1550 217 1333 Concentration N-Phosphonomethyl- 2.10% by14.60% by 650 ppm glycine weight weight Aminomethyl- 0.10% by 0.70% by 23 ppm phosphonic acid weight weight Glycine 0.20% by 0.30% by 0.18% byweight weight weight Bis(phosphono- 0.45% by 3.20% by  23 ppmmethyl)glycine weight weight NaCl/NH₄Cl 16.70% by 16.40% by 16.75% byweight weight weight

[0066] For the recovery of the N-phosphonomethylglycine, 50.0 g of theretentate and 30.0 g of water were added to a 250 ml round-bottomedflask having a Teflon blade stirrer. 14.24 g of 20% strength HCl wereadded dropwise to this at 40° C. in the course of 10 minutes until a pHof 1.3 was achieved. The resulting suspension was stirred at 40° C. fora further 3 hours and then allowed to cool to room temperature. Theprecipitated phosphonomethylglycine was filtered off with suction andwashed with 20 g of water. The solid was dried at 70° C. and 50 mbar for16 hours.

[0067] Yield: 4.10 g of solid (contains 94.7% of phosphonomethylglycinecorresponding to a recovery rate of 53%)

[0068] Analysis: NaCl: 0.0% Phosphonomethylglycine 94.7% (HPLC) Motherliquor: 106.8 g of solution

EXAMPLE 2 Batchwise Two-Stage Concentration of the Mother LiquorObtained as in Example 1

[0069] The pH of the mother liquor was adjusted to 4 as described inexample 1. The mother liquor was concentrated batchwise in a stirredpressure cell of the type described in example 1 having the samemembrane. The conditions were as described in example 1. The permeatewas fed to a further stirred pressure cell of the same type having thesame membrane and concentrated. The concentration was carried out at 40°C. and 40 bar. The mean specific permeate flow in the second stage is25.70 kg/m²h. The retention of N-phosphonomethylglycine calculated overboth nanofiltration stages is 99.99% and the depletion of chloride saltsis 77.82%. The results are compiled in table 2 below. TABLE 2 MotherRetentate Retentate liquor Stage 1 Stage 2 Permeate Amount (g) 1550 217133.3 1199.7 Concentration N-Phosphono- 2.10% by 14.60% by 6320 ppm  20ppm methylglycine weight weight Aminomethyl- 0.10% by 0.70% by  230 ppm— phosphonic acid weight weight Glycine 0.20% by 0.30% by 9630 ppm 930ppm weight weight Bis(phosphono- 0.45% by 3.20% by  230 ppm —methyl)glycine weight weight NaCl/NH₄Cl 16.70% by 16.40% by 16.40% by16.79% by weight weight weight weight

1. A process for the recovery of N-phosphonomethylglycine from anaqueous mixture which contains N-phosphonomethylglycine, ammoniumhalides, and alkali metal or alkaline earth metal halides and optionallyorganic impurities in dissolved form, where (a) the pH of the mixture isadjusted to a value in the range from 2 to 8, (b) a separation of themixture on a selective nanofiltration membrane is performed, a retentatewhich is richer in N-phosphonomethylglycine and poorer in halides and apermeate which is richer in halides and poorer inN-phosphonomethylglycine being obtained, and (c) theN-phosphonomethylglycine is recovered from the retentate.
 2. A processas claimed in claim 1, the mixture originating from the preparation ofN-phosphonomethylglycine, in which a triazine of the formula II

in which X is CN or CONR¹R², where R¹ and R² can be identical ordifferent and are H or C₁-C₄-alkyl, is reacted with a triacyl phosphiteof the formula III P(OCOR³)₃ in which the radicals R³ are C₁-C₁₈-alkylor aryl which is optionally substituted by C₁-C₄-alkyl, NO₂ orOC₁-C₄-alkyl, and the product obtained is hydrolyzed using a hydrohalicacid.
 3. A process as claimed in claim 2, a mixture being used which wasobtained as a mother liquor after reaction of the triazine of theformula II with the triacyl phosphite of the formula III, hydrolysis ofthe, product obtained using hydrochloric acid and separation of theN-phosphonomethylglycine at pH 0.5 to
 2. 4. A process as claimed inclaim 3, the mixture containing 0.5 to 3% by weight ofN-phosphonomethylglycine and 10 to 25% by weight of chloride salts.
 5. Aprocess as claimed in claim 1, the pH of the mixture being adjusted to avalue in the range from 2.5 to 6.5.
 6. A process as claimed in claim 1,a membrane having a separation limit in the range from 50 to 1000 Dbeing used in step (b).
 7. A process as claimed in claim 6, a membranehaving a separation limit in the range from 100 to 500 D being used. 8.A process as claimed in claim 1, the separation of the mixture in step(b) being carried out at a transmembrane pressure between retentate sideand permeate side in the range from 30 to 100 bar.
 9. A process asclaimed in claim 1, the separation of the mixture in step (b) beingcarried out at a flowing-over rate in the range from 0.1 to 5 m/s.
 10. Aprocess as claimed in claim 1, the separation in step (b) being carriedout in a number of stages by feeding the permeate from one stage to thesubsequent stage as a feed solution.
 11. A process as claimed in claim10, the retentate of the second or a further stage being at leastpartially fed to the first or the preceding stage.
 12. A process asclaimed in claim 1, the retentate obtained in stage (bl being subjectedto at least one diafiltration step.
 13. A process as claimed in claim 1,the permeate being treated with a strong base in order to release theammonia or amine contained in the ammonium halides.